The invention relates to a cladding tube for a fuel rod which is or can be used in a fuel element of a boiling water reactor, the cladding tube, between its inner side (the side facing toward the nuclear fuel enclosed in the cladding tube) (inner surface) and its outer side (outer surface), comprising a zirconium alloy with a practically constant chemical composition, but at these two surfaces having a different microstructure.
Such a fuel rod is illustrated in FIG. 1, in which the two ends of the cladding tube 1 are closed by means of metal stoppers 2 and enclose a column of fuel pellets 3. At least at one end (usually the top end), a spring 4 provides a gas collection chamber, while in the state immediately after it has been produced, there is a gap 5 between the pellets 3 and the cladding tube 1, which gap closes gradually, however, when the cladding tube is compressed by the pressure of the boiling water while the reactor is operating and the pellets swell. To ensure good heat transfer from the pellets to the cladding tube and the cooling water, the tube generally has a helium atmosphere of a few bar.
FIG. 1 also shows the pellets in the state 3a immediately after they have been produced and in the state 3b when the reactor has started to operate and the pellets have burst due to the high thermal loads.
In view of the fundamental requirement that in light water cooled nuclear reactors the cladding tubes for the fuel rods should exhibit low neutron absorption, the cladding tubes are made from a material which predominantly comprises zirconium of a purity which is standardized for tubes used in nuclear applications (e.g. R60001). However, in addition to the neutron absorption, a multiplicity of chemical, mechanical and other physical conditions which impose demands on the material and its production have to be observed, and some of these requirements are not compatible and, moreover, vary for different types of reactor (boiling water reactor or pressurized water reactor). When used for long periods in water or steam, pure zirconium is not sufficiently corrosion-resistant and must therefore be weakly alloyed with additions which have to be adapted according to the type of reactor.
Thus, the nuclear reaction causes iodine and other gaseous fission products to be formed in the nuclear fuel, leading, on the one hand, to an increase in volume of the fuel and, on the other hand, to an aggressive atmosphere on the inner side of the cladding tube. The pellet fragments 3b (FIG. 1) may lead to punctiform pressure and substantial local stresses on the inner surface of the cladding tube and, at the same time, the aggressive fission products are directed onto the inner surface through the fractured surfaces. In zircaloy, which is the standard material for cladding tubes, this combination of local stresses and an aggressive atmosphere results in stress cracks beginning to form from the contact points, along which stress cracks intensified corrosion propagates, initiated primarily by the iodine. These stress corrosion cracks grow through the entire wall thickness of the cladding tube and lead to perforation of the cladding tube (so-called xe2x80x9cpellet cladding interactionxe2x80x9d, PCI).
Pure zirconium (e.g. xe2x80x9csponge zirconiumxe2x80x9d, which is the standard commercially available form of reactor-purity zirconium) is less susceptible to PCI, since pure zirconium has a higher ductility than zircaloy, so that the local stresses are partially absorbed by plastic deformation of the zirconium and are therefore unlikely to reach the threshold which is critical for PCI. However, pure zirconium is too soft in terms of the high mechanical stability required of such cladding tubes (diameter: approx. 1 cm, length approx. 4 m, wall thickness approx. 1 mm!). For this reason, so-called xe2x80x9cliner cladding tubesxe2x80x9d, in which a tube made from zircaloy has a thin lining of pure zirconium on the inner side, are frequently used. Since the introduction of such liners, punctiform damage caused by PCI is scarcely ever observed on the corresponding cladding tubes.
Zircaloy is a standardized alloy (e.g. US standard R60802) which has as far as possible been optimized in terms of stability by the addition of tin and in terms of corrosion by the addition of iron, chromium and, if appropriate, nickel.
However, PCI damage has been observed practically only in boiling water fuel elements, but not in pressurized water fuel elements, even though the high pressures in the pressurized water reactor press the cladding tube onto the fuel over the course of time (the so-called xe2x80x9ccreepxe2x80x9d phenomenon). However, the particular way in which boiling water reactors are controlled results in particularly high loads. The most common cause of damage in pressurized water fuel rods is chemical corrosion from the water which attacks the outer surface and/or mechanical corrosion caused by friction in the fuel element (so-called xe2x80x9cfrettingxe2x80x9d). In this case, aqueous corrosion acts practically uniformly on the entire surface of the cladding tube, which is therefore attacked uniformly (uniform corrosion), this corrosion behavior being considerably intensified by the high operating temperature and the chemical composition of the pressurized water in the pressurized water reactor.
Due to the lower operating temperature and the water in the boiling water reactor containing more oxygen, in practice the corrosion observed on the outer surface of the cladding tubes in such reactors is not uniform, but rather is characterized by punctiform, locally delimited oxide blisters (so-called xe2x80x9cnodular corrosionxe2x80x9d), which are not observed in the pressurized water reactor. While individual blisters are often tolerable, a denser covering with these blisters may lead to deposition (so-called xe2x80x9ccrudxe2x80x9d) of contaminants and dissolved metals (e.g. copper) from the boiling water, an effect which reduces the cooling of the fuel rods and, in extreme cases, uniform corrosion may also be considerably accelerated as a result of overheating of the fuel rod.
Nowadays, the cause of the nodular corrosion is considered to be the fact that the alloying elements iron, chromium and nickel are deposited as secondary phases in zirconium alloys, i.e. as particles (xe2x80x9csecondary phase particlesxe2x80x9d, SPPS) which are distributed throughout the entire grain structure of the material and the number, size and spacing of which are determined by the manufacturing process. If these SPPs have become too large owing to high manufacturing temperatures, they initiate nodular corrosion under the aqueous-chemical conditions of the boiling water reactor. For this reason, cladding tubes for boiling water reactors are manufactured in a xe2x80x9clow-temperature processxe2x80x9d (LTP).
However, advances in reactor engineering have led to the fuel containing ever more fissile material, i.e. having a higher energy content, thus allowing a longer service life (so-called xe2x80x9cburn-upxe2x80x9d) of the fuel rods and also leading to somewhat higher fuel-rod and operating temperatures. It is therefore necessary even in boiling water reactors to take into account uniform corrosion of the cladding tubes, which according to current knowledge is promoted if the size of the SPPs is too small. Therefore, there is a need for manufacturing processes which allow optimization between nodular and uniform corrosion.
Further damage to cladding tubes is formed by cracks which have a considerable extent in the axial direction. Although these extensive cracks are significantly less common than the PCI defects mentioned above, they also lead to significantly greater disruptions to operation, since significant quantities of the fuel rod contents can be washed out through these cracks. Since these cracks occur considerably more often in liner cladding tubes than in liner tubes which consist entirely of zircaloy (so-called xe2x80x9csolid-wall tubesxe2x80x9d), there are increasing objections to the use of the pure zirconium liner. Moreover, in the case of the liner tubes, it is necessary to ensure, by means of meticulous quality testing, that the liner adheres firmly to the supporting tube, so that there can be no disruption to the dissipation of heat resulting in corresponding local overheating of the fuel rod.
The invention is therefore based on the object of providing and producing a single-component cladding tube which, on the inner surface, has a high resistance to PCI and to the extended cracks mentioned above, which are attributable to embrittlement, and, at the same time, on the outer surface is as resistant as possible both to uniform corrosion and to the nodular corrosion which arises in the cooling water of the boiling water reactor.
The invention works on the basis that the stress-corrosion cracking (induced primarily by iodine), which is largely independent of the precipitated secondary phases, can be practically prevented by a microstructure of the matrix in which an optimum grain size is combined with an optimum texture. This microstructure is therefore to be approximately of the same ductility as the iodine-resistant, ductile zirconium liner and, at the same time, is also to be resistant to the extended cracks which are formed in the liner; i.e. it should not be damaged extensively either by corrosion or by embrittlement.
However, the inner surface should also have a better resistance to uniform corrosion than that of pure zirconium, because small amounts of water may penetrate through slight defects which in themselves are tolerable (e.g. undiscovered, small leaks in the weld seams or those brought about by the xe2x80x9cfrettingxe2x80x9d phenomenon mentioned above) into the interior of the cladding tube, where this water reacts with the wall material and the fuel so as to evolve oxygen; the oxidation reaction would be insignificant, but the resultant hydrogen would not be, since it would make the wall material brittle. The interaction of oxidation and embrittlement resulting from the uptake of hydrogen may then lead to the cracks mentioned above.
Therefore, according to EP-A-0,726,966 the pure zirconium of the liner is alloyed with about 0.5% by weight of iron, which is practically insoluble in zirconium. The iron is precipitated in the form of particles which increase the resistance to uniform corrosion but only bring about a slight dispersion hardening, i.e. scarcely change the ductility of the pure zirconium.
However, a composite tube of this nature is expensive to produce, since it is necessary to avoid the risk of manufacturing errors. Therefore, there is a desire for solutions which allow a material of uniform chemical composition to be used while satisfying the different requirements imposed on the two surfaces by means of differences in the microstructure, i.e. in the grain structure of the alloy matrix and/or the form and distribution of secondary phases in which insoluble alloying fractions are precipitated.
For example, it is proposed in DE-A-29 51 102 for the outer surface of a zircaloy cladding tube to undergo secondary heating with laser beams in the xcex2-area and to be cooled rapidly, in order in that area to establish a quenched xcex2-structure of the matrix with particularly small grains. According to GB-B-1,529,664, a similar effect is achieved by the finished tube being heated again from the outside, while the inner surface is kept at a lower temperature by means of a flow of water (xe2x80x9ctemperature gradient annealingxe2x80x9d).
According to EP-A-0,660,883, the outer surface is heated into the b-range and is then cooled, but the inner surface is held at a moderately elevated temperature (xe2x80x9cpartial xcex2-quenchingxe2x80x9d), water being atomized onto the inner surface by means of hot inert gas, in order to limit the temperature gradient during xcex2-quenching. Then, the quenched xcex2-structure is present in a relatively wide layer on the outer surface, while the xcex1-structure is present in a thin inner layer which in practice constitutes a liner.
According to U.S. Pat. No. 4,718,949, the partial xcex2-quenching can also be combined with temperature gradient annealing. In that document, it is proposed for the outer surface of a tubexe2x80x94before or after pilgering steps which are used to produce the final dimensions of the tubexe2x80x94to be heated to the xcex2-range, while the inner surface is being cooled. As a result, to protect against nodular corrosion, the alloying constituents at the outer surface are to be held predominantly in the matrix and there is to be less precipitation than on the inner side. Then, the outer surface is cooled and the inner is annealed at the recrystallization temperature in the xcex1-range. However, these measures require long processing times and a high outlay on equipment in order to keep the entire length of the finished cladding tube in the temperature range required for a sufficiently long time, and are therefore not employed.
However, the invention works on the basis that at least the areas on the inner wall, and preferably practically all the areas of the tube wall, should have a high ductility, in order to reduce not only the formation, but also the propagation, of stress cracks. This ductility can be achieved by a matrix of particularly small grains, although larger grains are also possible given a specific structure of the grains, which can be described by a relatively high Kearns factor. This leads to the introduction of a ductility parameter xcex3=3{square root over ((KD))}/(fr)2, where (KD) is the mean grain diameter, measured in xcexcm, and (fr) is the Kearns factor. By means of a thermal/mechanical processing which has a virtually uniform action on all parts of the tube (i.e. eliminates temperature gradient annealing), the two variables can be set in such a way that xcex3 less than 3.5. This corresponds to an elongation at break for the material which is over about 20% at 300xc2x0 C.
The chemical composition of the material is selected with a view to corrosion; the composition of zircaloy-2 or zircaloy-4 is particularly suitable; alternatively, a composition containing the same alloying constituents but in concentrations which are optimized so that they deviate slightly from the standards for zircaloy may also be suitable. As has already been mentioned, the same chemical composition may lead to high nodular corrosion and low uniform corrosion or to the opposite scenario if the size and amount of precipitation of undissolved alloying constituents (xe2x80x9csecondary phasesxe2x80x9d) are changed by the heat treatment during manufacture.
The invention therefore provides for the cladding tube to be manufactured with a practically homogenous chemical composition, but for the distribution (size and amount) of secondary phases to be matched to the requirements imposed on the inner surface and the outer surface by manufacturing these surfaces from a material which undergoes different preliminary heat treatments prior to the abovementioned thermal/mechanical treatment which acts evenly on all areas of the cladding tube and is used to attain the final dimensions of the tube.
This is because the invention provides for the inner surface to have a certain minimum covering of particles of a certain minimum size in order to protect against uniform corrosion and the cracks mentioned above. In this context, however, it is inevitable that particles of this minimum size will also be formed at the outer surface. The inevitable presence of large particles on the outer surface is, however, contrary to the requirement in that area that, with regard to nodular corrosion, the size and number of particles on the outer surface are to be limited.
However, the invention provides a manufacturing process which makes it possible to produce a cladding tube which fulfills these contradictory requirements.
The object set is therefore solved by means of a process and a cladding tube. Advantageous refinements of the invention are described in the subclaims.